Ruminant or industrial sources of trans fatty acids: public health issue or food label skirmish?

In this issue of the Journal, 2 groups of investigators report carefully conducted controlled feeding studies in healthy volunteers that compared the effects on blood lipids of trans fatty acids (TFA) from industrial (partially hydrogenated vegetable oil) and from dairy (ruminant or “natural”) sources. Chardigny et al (1) compared industrially derived and ruminant-derived TFA at 5% of energy. Motard-Belanger et al (2) compared industrially derived and ruminant-derived TFA at 3.7% of energy and also included a third arm with only 1.5% of energy from ruminantderived TFA and a control arm with 0.8% of energy from TFA. In the study of Chardigny et al, the industrial TFA diet lowered HDL-cholesterol, LDL-cholesterol, and triacylglycerol concentrations compared with dairy TFA only in women; in men, significant differences between the TFA diets were not seen. The ratio of total to HDL cholesterol and the concentration of lipoprotein(a) [Lp(a)] also tended to be higher with the industrial TFA diet, but these differences were not significant. As the authors noted, a control arm replacing TFA with nonhydrogenated fats would have made the study more informative. Fortunately, Motard-Belanger et al had the equivalent of 2 control arms, comparing high-industrial and high-ruminant TFA diets (3.7% of energy) with both a lower-ruminant TFA diet (1.5% of energy) and a low total TFA diet (0.8% of energy). Compared with the 2 lower-TFA diets, the diets with 3.7% of energy from TFA, whether from ruminant or industrial sources, each had similar adverse effects on blood lipids and lipoproteins, including increases in LDL cholesterol, decreases in HDL cholesterol, and increases in the ratio of total to HDL cholesterol (although power to achieve statistical significance for some comparisons was limited because of dropouts). However, consumption of 1.5% of energy from ruminant TFA did not have significantly different effects on blood lipids or lipoproteins than did consumption of the low-TFA (0.8% of energy) diet. The interpretation of both studies is somewhat limited because, as in most of the earlier controlled feeding trials of TFA, the durations of each diet were relatively brief (3–4 wk), the diets also contained different amounts of some specific saturated fatty acids, and statistical power to detect some differences may have been limited. In addition, participants were selected to be leaner, younger men and women, whereas those at risk of heart disease typically are overweight persons who are middle-aged or older. Because the metabolic effects of TFA are similar to several features of the metabolic syndrome (3), it is worrisome that adverse effects could be underestimated in healthier persons and more prominent in persons with greater underlying insulin resistance. These 2 studies are the first controlled feeding studies to directly compare industrial with “natural” TFA. Others have found that feeding dairy products enriched with ruminant-derived TFA, compared with control diets, has adverse effects on blood lipids (4, 5); the changes were qualitatively similar to those produced by industrial TFA in other studies. In one trial, a butter enriched with natural TFA increased total cholesterol less than did a control butter, but the ratio of total to HDL cholesterol was not significantly different (6). As both of the present reports acknowledge, the amount of TFA from dairy sources used in these studies greatly exceeded the intake of ruminant TFA in usual diets. Bacteria in the stomachs of ruminants (eg, cattle, sheep, and goats) can biohydrogenate the relatively small amounts of polyunsaturated fatty acids present in ruminant feed to form TFA isomers, the most abundant of which is an 18:1 trans isomer, vaccenic acid. These TFA, incorporated into milk and beef fat, typically constitute 5% of the total fatty acids. Thus, if a person were to consume the entire maximum recommended saturated fat intake (10% of energy) from ruminant sources, that would correspond to 20% of energy from ruminant fats (given that ruminant fats are roughly 50% saturated fatty acids), and the intake of “natural” TFA would be 5% of that amount, or 1% of total energy. Because nonruminant sources also significantly contribute to saturated fat intake in most diets, ruminant TFA consumption in most persons is substantially lower than 1% of energy, and far smaller than the 3.7% and 5% (and even the 1.5%) used in these trials. Although Chardigny et al did not describe the methods used to obtain the high amounts of TFA in their dairy products, the investigators presumably used methods similar to those of Motard-Belanger et al—ie, feeding cows large amounts of polyunsaturated oils, which is certainly not what is “naturally” done. Whether equal amounts of TFA from industrial and ruminant sources have similar effects on blood lipids is an interesting scientific question, but it does not appear to be a critical public health or policy issue. The relation of TFA from natural versus